1. Field of the Invention
The present invention relates to generation of optical pulse trains, and in particular to an apparatus and method for generating an optical pulse train having a high repetition rate.
2. Description of the Related Art
Optical telecommunications systems and optical computers often require optical pulse trains having a high repetition rate.
Modulated mode-locked semiconductor lasers have been recognized as sources of optical pulse trains having a high repetition rate. A modulated mode-locked semiconductor laser is disclosed in U.S. Pat. No. 6,031,851, which is incorporated herein by reference in its entirety. FIG. 1 is a schematic of the conventional modulated mode-locked semiconductor laser. The conventional modulated mode-locked semiconductor laser is composed of a substrate 55, an active region 56 and electrodes 57-59. The substrate 55 and active region 56 are divided into a distributed Bragg reflector section (DBR section) 51, an electroabsorption modulator section 52, a gain section 53 and a saturable absorber section 54.
Generation of an optical pulse train is achieved by using the conventional modulated mode-locked semiconductor laser as described in the following. The saturable absorber section 54 is provided with a reverse bias voltage through the electrode 57. The gain section 53 is provided with a dc current through the electrode 58. The electroabsorption modulator section 52 is provided with a modulation bias through the electrode 59, the modulation bias being generated by superposition of a reverse dc bias voltage and a sinusoidal voltage. The modulation bias causes an optical pulse train to be emitted. The conventional modulated mode-locked semiconductor laser can generate an optical pulse train having a repetition rate in the range from 30 to 40 GHz.
Generation of optical pulse trains is also achieved by using an array waveguide diffraction grating (AWG) or a Fabry-Perot etalon. FIG. 2 shows an AWG apparatus 71 for generating an optical pulse train. The AWG apparatus 71 is composed of an input waveguide 61, an optical demultiplexer 62, a plurality of waveguides 63, an optical multiplexer 64, and an output waveguide 65. The optical path lengths of the plurality of waveguides 63 are different from each other. The input waveguide 61 is provided with an optical pulse train Pin as shown in FIG. 3A. As shown in FIG. 2, the optical pulse train Pin is demultiplexed by the optical demultiplexer 62 to generate a plurality of be optical pulse trains. The generated optical pulse trains are respectively supplied to the waveguides 63 to be delayed. The different optical path lengths of the waveguides 63 causes the optical pulse trains to be delayed by different delay times. The delayed optical pulse trains are multiplexed by the multiplexer 64 to generate an output optical pulse train Pout. The output optical pulse train Pout has a repetition rate m times that of the input optical pulse train Pin, N being the number of the waveguides 63.
Yokoyama et al. disclose still another technique for generating optical pulse trains in Japanese Laid Open Patent Application (Jp-A Heisei 8-148749). The optical pulse train generating technique uses two semiconductor lasers, one of which is a modulated mode-locked semiconductor laser, and the other is a passive mode-locked semiconductor laser. The modulated mode-locked semiconductor laser generates an optical pulse train to excite the passive mode-locked semiconductor laser. The passive mode-locked semiconductor laser generates an output optical pulse train in synchronization with the optical pulse train from the modulated mode-locked semiconductor laser. The synchronization achieved by the optical excitation improves the stability of the repetition rate of the output optical pulse train.
Resent development of optical signal processing increases a demand of generation of optical pulse trains having a repetition rate higher than 50 GHz, desirably higher than 80 GHz, while the repetition rate is precisely controlled. A technique enabling the generation of optical pulse trains having precisely controlled high repetition rates has been desired.
Therefore, an object of the present invention is to provide an optical pulse train generator for generating optical pulse trains having high repetition rates.
Another object of the present invention is to provide an optical pulse train generator for precisely controlling a repetition rate of a generated optical pulse train.
An optical pulse train generator includes first and second optical resonators and an optical pulse generator. The first optical resonator has therein a first optical path of a first optical path length. The second optical resonator is inserted into the first optical path. The second optical resonator has therein a second optical path of a second optical path length substantially equal to 1/m of the second optical path length, m being a natural number. The optical pulse generator provides the first optical resonator with a first optical pulse train including a component of a first repetition rate tuned corresponding to the first optical path length. The provision of the first optical pulse train allows a second optical pulse train to be generated in the first optical resonator. The first optical resonator extracts a portion of the second optical pulse train to output an output optical pulse train.
When the optical pulse train generator further includes a laser gain element inserted into the first optical path of the first optical resonator, the laser gain element preferably receives the first optical pulse train for amplifying the second optical pulse train in synchronization with the first optical pulse train.
The laser gain element is preferably a mode-locked semiconductor laser including a saturable absorber region receiving the first optical pulse train, and a gain region coupled to the saturable absorber region to amplify the second optical pulse train.
It is preferable that the mode-locked semiconductor laser has a reflecting surface on the saturable absorber region, and the first optical resonator includes a reflecting mirror, the first optical path being formed between the reflecting surface and the mirror through the gain region and the second optical resonator.
The reflecting mirror is preferably movable to enable to adjust the first optical path length.
The first optical resonator may be formed of a Fabry-Perot resonator having a pair of reflecting surfaces between which the first optical path is formed,
In this case, the first repetition rate of the first optical pulse train is preferably f/n, n being a natural number and f being a circumferential frequency defined by:
f=c/2L,
where c is the velocity of light in vacuum, and L is the first optical path length of the first optical path between the pair of reflecting surface. This causes the second repetition rate of the second optical pulse train to be m times the circumferential frequency f.
The second optical resonator may be formed of a Fabry-Perot etalon including a pair of optical flats between which the second optical path is formed.
In this case, the Fabry-Perot resonator and the Fabry-Perot etalon are preferably designed not to form a resonator between the Fabry-Perot resonator and the Fabry-Perot etalon. This is preferably achieved by that the pair of optical flats is oblique to the first optical path formed between the pair of reflecting mirrors of the Fabry-Perot resonator.
The first optical resonator may be formed of a ring resonator, the first optical path of the first optical resonator being looped.
In this case, the first repetition rate of the first optical pulse train is preferably fxe2x80x2/n, n being a natural number and fxe2x80x2 being a circumferential frequency defined by:
fxe2x80x2=c/L,
where c is the velocity of light in vacuum, and L is the first optical path length of the looped first optical path. This causes the second repetition rate of the second optical pulse train to be m times the circumferential frequency fxe2x80x2.
The first optical resonator may include an optical fiber to form the ring resonator.
The second optical resonator may be formed of a Fabry-Perot etalon, while the first optical resonator is formed of a ring resonator.
When the first optical resonator is formed of a ring resonator, the second optical resonator may be formed of another ring resonator, the second optical path of the second optical resonator being looped.
The second optical resonator includes another optical fiber to form the other ring resonator.
The first and second optical path lengths of the first and second optical resonators are preferably adjustable.